The metabolism and enzymology of the transforming protein, p21, of Harvey and Kirsten sarcoma virsues have been investigated. A precursor polypeptide to the p21 has been detected and two processed product forms of the protein chased from the precursor. Tentative evidence has been obtained that the p21 is a subunit of a membrane associated phosphokinase. A unique hemopoietic precursor cell line has been identified which expresses high levels of an endogenous p21, suggesting that p21 metabolism plays a role in controlling proliferation of hemopoietic stem cells.